The PSU that you have come to know in your desktop is an ATX PSU but that is not the only choice, as server techs and SFF builders are well aware. One of the alternative standards is the TFX which retains the ATX electrical and connection specs but fits it into a 5.75" × 3.25" × 2.5" (146 x 83 x 64mm) casing. The 350W Seasonic SS-350TGM is not designed for SLI/CrossFire systems, indeed it does not have a single PCIe connector of any type which is not a problem for the type of machine this is intended for. If you are planning on building an ITX box with an internal PSU, or even a small custom built case, you really should drop by [H]ard|OCP to check this little PSU out.

"Today we tackle something a bit off the beaten path for us; a low power non-ATX specification power supply from Seasonic. We do know that many of our readers build systems that are not like all the others especially when it comes to crafting smaller purpose driven boxes. The 300TGM may be just what you are looking for."